1. Field of the Invention
The invention relates to a steering system and, more particularly, relates to a steering system for a self-propelled lawnmower.
2. Discussion of the Related Art
Lawnmowers equipped with dual hydraulically-powered drive systems that propel the drive wheels on each side of the mower are known in the grooming a lawns. Turning is achieved by driving each drive wheel at different speeds. They can even be turned in different directions for a very sharp or even a “zero-turn” radius, where the mower pivots about the midpoint drive wheel axis. This kind of drive wheel steering system is more maneuverable than traditional vehicle steering systems that employ one set of wheels, either front or rear, which turned in unison while other drive wheels are powered to drive the mower in a forward or rearward direction.
Hydraulically-powered drive systems typically include a separate variable displacement hydraulic pump for each of the drive wheels. Each variable displacement pump is typically an axial-piston type which includes a tilting swash plate which can vary the pump discharge rate from a zero flow, also referred to as neutral, up to a maximum flow or reverse direction flow to a particular maximum.
The pumps are controlled by an operator controlled steering mechanism. Several such mechanisms are known, all of which have disadvantages.
For example, U.S. Pat. No. 5,822,961, entitled “Quick Adjustment for Straight Ahead Travel for a Wheel-Steered Lawn Mower,” discloses a pistol-grip type of steering mechanism for adjusting a dual-hydraulically-powered walk-behind mower. Right and left steering control levers can be squeezed to control steering of the walk-behind mower.
A drawback of this type of steering control mechanism is that it requires squeezing with the operator's fingers to maneuver the mower. Repeated squeezing motions causes fatigue and increases the likelihood of carpal tunnel syndrome. Vibrations are also transmitted to the operator's hands.
Another drawback is that this type of steering control is not adjustable for variable sizes of hands. The physics of operating larger machinery requires larger steering linkages, which are more difficult to operate with smaller hands. This further increases fatigue and the likelihood of carpal tunnel syndrome.
U.S. Pat. No. 5,913,802, entitled “Single Lever Drive Wheel Steering Power Lawn Mower,” discloses a self-propelled mower equipped with a pair of variable flow hydraulic pumps controlled by a T-shaped steering handle. The T-shaped steering handle is configured to rotate about a vertical axis so as to transmit control forces to the variable displacement pumps so as to control the steering of the lawn mower. The handlebar can also be rotated or twisted about a horizontal axis to control the speed of the mower from a stationary neutral position or a reverse direction position. This steering control mechanism has similar drawbacks to that described above for the pistol-type grip lever. Repeated twisting motions with the wrists to control steering of the mower causes fatigue and increases the likelihood of carpal tunnel syndrome. Another drawback is the difficulty encountered in simultaneously twisting the grip and rotating a grip handle to maneuver a turn of the mower.
In addition, speed limiters for traditional lawnmowers must be operated independently of the steering system, complicating operation and making the lawnmower more difficult to control during a turn.